In the hydrocarbon exploration and production industry, it is common to position tools or toolstrings in a wellbore to perform intervention or workover operations. Wireline or slickline interventions convey the tool or toolstring from a flexible cable-like line which is controllably deployed from a powered winch.
Accurate positioning of tools within a wellbore is a commonly accepted limitation of wireline well intervention operations. The depth of a typical well, combined with the changes in deviation and azimuth along its length, mean that it may be difficult to calculate the position of the tools based simply on the length of the cable in the well to an accuracy within one or two feet (about 0.3 m to 0.6 m) using a distance encoder on the winch. One method used to improve accuracy involves electronically detecting joints between lengths of wellbore casing, the joints being at known positions. The winch encoder is then used to measure incremental depths from the last (or a recently passed) joint. However, this technique cannot reliably position a tool to within an accuracy of within one foot (around 0.3 m).
Inertial navigation tools such as those which use combinations of GPS receivers, accelerometers, gyroscopes, magnetometers and/or pressure sensors, may be able to improve on this accuracy, but they are expensive and not appropriate for the majority of well intervention operations.
The difficulties mentioned above have led to the development of equipment which utilise features in the wellbore to position a tool. These features may be designed into the completion for the express purpose of positioning tools. In this case, the completion will be designed to interact with a component carried on the toolstring to be positioned, so that the toolstring will not pass the wellbore feature. For example, the wellbore feature may simply be a restriction which is too small for a tool of a given diameter to pass. Alternatively, the wellbore feature may comprise a special profile so that a spring-loaded mechanism with a matching profile on the tool will automatically engage the profile as the tool passes.
Generally speaking, these types of wellbore features will only be positioned at a few specific points in the well and it is very unlikely that they will be of use for wireline interventions that had not been anticipated by the well designers. The lifetime of a well completion may be up to 20 years, and it is likely that new technology and intervention techniques will come into use between the design of the well and its ultimate abandonment.
It is also possible to utilise particular wellbore features that are designed into the wellbore completion for some other reason (other than tool positioning). However, as these wellbore features have not been designed with tool positioning in mind, the engagement mechanisms used may need to be relatively complex to effectively locate on them. In one typical scenario, a wellbore restriction of a first inner diameter, on which a sub-assembly of the toolstring is desired to locate, may be positioned beneath another restriction of second inner diameter, less than the first. In this situation, a subassembly which is able to pass the upper restriction of lesser inner diameter is not able to locate on the lower restriction.
Intervention tools have been proposed which use simple clamping mechanisms to position the tool on the inside of the casing. However, such tools do not address issues of accurate tool positioning.
One intervention technique that requires accurate tool placement is the use of electric cutting tools. These electric cutting tools provide a clean and controlled cut of downhole tubulars, and in one application are can be used to cut a sleeve inside a packer so as to allow it to be released and safely removed from a well. However in order to work the cutting tool must be positioned to an accuracy within 6 inches (around 0.15 m). This is often not within the operational capabilities of available intervention equipment, and in some applications it becomes necessary to adopt a different intervention approach requiring more rig time and expense.
There is generally a need for an apparatus for positioning a tool or toolstring within a wellbore and a method of use which addresses one or more of the problems identified above, and/or obviates or mitigates one or more drawbacks or disadvantages of the prior art.
In particular, one aim of an aspect of the invention is to provide an apparatus for positioning a tool or toolstring at a restriction in a wellbore, which is lower in the wellbore than a smaller restriction through which the apparatus is able to pass, and a method of use. It is an aim of an aspect of the invention to provide an assembly for cutting a downhole tubular incorporating a positioning apparatus and a method of use. Further aims and objects of the invention will become apparent from reading the following description.